A standard was established in 2002 for Blu-ray Disc (hereinafter referred to as BD), which has high density and high-capacity, and on and from which information is recorded and reproduced using a blue-violet laser, as a medium for recording high-definition images. There are two types for BD: 25 GB capacity single-layer media (with one information layer on one side); and 50 GB capacity dual-layer media (with two information layers on one side). Particularly, in the dual-layer media, the translucent information layer located closer to a laser beam incident side is called Layer 1 (hereinafter referred to as L1), and the information layer located farther from the laser beam incident side is called Layer 0 (hereinafter referred to as L0).
The present inventors developed a rewritable BD (hereinafter referred to as a BD-RE) medium and put into practical use a 1× speed 25 GB capacity medium and a 1× speed 50 GB capacity medium in 2004. 1× speed corresponds to a data transfer rate of 36 Mbps. Furthermore, the present inventors put to practical use a 2× speed 25 GB capacity medium and a 2× speed 50 GB capacity medium in 2006. These media allow the data and image information to be recorded (amorphous state) and rewritten thereon and erased (crystalline state) therefrom by phase change of their recording layers. The material and the composition of the recording layer is determined so as to obtain an optimal crystallization rate to the speed of the medium.
The present inventors used a Ge—Sb—Te recording material (see JP 2584741 B, for example) for 1× speed media, and a Ge—Bi—Te recording material (see JP 2574325 B and WO 2006/011285, for example) having a higher crystallization rate for 2× speed media. More specifically, the Ge—Sb—Te recording material used was a compound obtained by combining GeTe with Sb2Te3, having a composition containing approximately 44% of Ge and approximately 6% of Sb. The Ge—Bi—Te recording material used was a compound obtained by combining GeTe and Bi2Te3, having a composition containing approximately 45% of Ge and approximately 4% of Bi.
Further increases in the speed and capacity of BD will make BD more valuable and useful for personal computer, recorder, and game machine applications in the future. For example, a higher processing speed for data and image files, higher definition, improved sound quality, and additional recorder functions can be obtained. Also, a longer recording time and substitution for hard disks can be realized. Assuming that the demand for high-speed, high-capacity media will increase further in the future, the inventors set the next development goals to producing a medium usable at a speed of at least 4× speed (144 Mbps) with a capacity of at least 100 GB.
To produce a 4× speed medium requires a material with a higher crystallization rate than that of the recording material used for a 2× speed medium. For example, the Ge—Bi—Te recording material has a higher crystallization rate when the concentration of Bi2Te3 therein is increased. Particularly, in the 50 GB capacity dual-layer medium, the recording layer L1 is as thin as 6 nm while the recording layer L0 has a thickness of approximately 10 nm. Thus, when recording materials with the same composition are used for the L0 and L1, the L1 has a lower crystallization ability than that of the L0. Therefore, it is necessary to increase the concentration of Bi2Te3 in the recording material for L1.
On the other hand, a high capacity of 100 GB can be attained by, for example, increasing the recording density by two times, and by increasing the number of the information layers to four. Increasing the recording density requires recording marks to be small, resulting in a shorter irradiation time of the laser beam. In order to be crystallized in a short time, the recording material needs to have a high crystallization rate. In the case of increasing the number of the information layers, it is necessary to add another information layer having a higher transmittance than that of the L1 of the dual-layer medium. Accordingly, the recording layer is designed to have an extremely small thickness, for example, 3 nm. Since the crystallization rate needs to be increased also for increasing the capacity as mentioned above, it is necessary to adjust the composition of the Ge—Bi—Te recording material by increasing the concentration of Bi2Te3 therein to be higher than that of the recording material for the L1 of the dual-layer medium.
First, in order to obtain a 4× speed dual-layer medium, the present inventors looked for an optimum value of Bi2Te3 concentration in the Ge—Bi—Te recording material, but failed to find it either for the L0 or for the L1. The present inventors evaluated the L0 and the L1 for initial properties (recording properties and erasing properties) and reliability (recording mark storage stability). The result on the L0 was that when the L0 was composed of a composition with satisfactory erasing properties, it exhibited a poor recording mark storage stability and was not usable practically. The result on the L1 was that when the L0 was composed of a composition with satisfactory erasing properties, it had a small signal amplitude and insufficient recording properties. Moreover, since the L1 exhibited a poor recording mark storage stability like L0, no optimal composition could be found for it.
These results are derived from the fact that in the Ge—Bi—Te recording material, a higher Bi2Te3 concentration increases the crystallization rate, but at the same time, it reduces the optical variation and lowers the crystallization temperature. The crystallization rate is evaluated as the erasing properties, and when the crystallization rate is high, an erasure rate is high. The optical variation is a refractive index variation caused by the phase change and is evaluated as the signal amplitude. When the optical variation is large, the signal amplitude is high. The crystallization temperature affects the recording mark storage stability, that is, the stability of the amorphous state. A decrease in the crystallization temperature leads to a deterioration of the recording mark storage stability. For the 2× speed medium in practical use, there was a composition capable of realizing all of satisfactory level of the crystallization rate, the optical variation, and the crystallization temperature, allowing the medium to exhibit satisfactory initial properties and reliability. In order for the medium to be usable at 4× speed, the Bi2Te3 concentration needed to be increased further. However, when the Bi2Te3 concentration was increased, the medium was not usable practically in terms of the amount of the optical variation and the crystallization temperature.
Likewise, in the experiment in obtaining a high capacity with a four-layer medium (100 GB, with four information layers on one side), there could not be found a composition with the satisfactory levels of the crystallization rate, the optical variation, and the crystallization temperature for all of the four information layers. When the four information layers were composed of a composition with satisfactory erasing properties, they exhibited poor recording mark storage stability. Also, the signal amplitude was insufficient on the translucent information layer.